I donīt know when are you going to Lisbon, but my sister has
had good experiences looking for hotels using the Bancotel
website: http://www.bancotel.com

A few times she has been able to get the Marriott or the
Holiday Inn for only one voucher per night ( 50 Euros ).
Granted, they are not in the centre and it doesnīt include
breakfast ... but we are talking 4 star here, and public
transport in Lisbon is quite good. :)

If you decide to go the Bancotel way, you should know that
many times you can preorder the vouchers to be picked up at
the hotel you are staying.